Re: [RFC] Ext3 online defrag

[Dave Kleikamp <shaggy@austin.ibm.com>]

On Tue, 2006-10-24 at 23:59 +1000, David Chinner wrote:
> On Tue, Oct 24, 2006 at 12:14:33AM -0400, Jeff Garzik wrote:
> > On Mon, Oct 23, 2006 at 06:31:40PM +0400, Alex Tomas wrote:
> > > isn't that a kernel responsbility to find/allocate target blocks?
> > > wouldn't it better to specify desirable target group and minimal
> > > acceptable chunk of free blocks?
> > 
> > The kernel doesn't have enough knowledge to know whether or not the
> > defragger prefers one blkdev location over another.
> > 
> > When you are trying to consolidate blocks, you must specify the
> > destination as well as source blocks.
> > 
> > Certainly, to prevent corruption and other nastiness, you must fail if
> > the destination isn't available...
> 
> That's the wrong way to look at it. if you want the userspace
> process to specify a location, then you should preallocate it first
> before doing anything else. There is no need to clutter a simple
> data mover interface with all sorts of unnecessary error handling.

You are implying the the 2-step interface, creating a new inode then
swapping the contents, is the only way to implement this.
> 
> Once you've separated the destination allocation from the data
> mover, the mover is basically a splice copy from source to
> destination, an fsync and then an atomic swap blocks/extents operation.
> Most of this code is generic, and a per-fs swap-extents vector
> could be easily provided for the one bit that is not....

The benefit of having such a simple data mover is negated by moving the
complexity into the allocator.

A single interface that would move a part of a file at a time has the
advantage that a large file which is only fragmented in a few areas does
not need to be completely moved.

> The allocation interface, OTOH, is anything but simple and is really
> a filesystem specific interface. Seems logical to me to separate
> the two. 

So what then is the benefit of having a simple generic data mover if
every file system needs to implement it's own interface to allocate a
copy of the data?

Shaggy
-- 
David Kleikamp
IBM Linux Technology Center